Pneumatic rubber passenger and truck tires are composed of elements which conventionally include a tread of a rubber composition. The tread rubber is sometimes desirably compounded to provide a tire with a relatively low rolling resistance with reasonable wear and traction.
Although it is desired to compound the tire's tread composition to reduce the rolling resistance of the tire without substantially degrading the tire's traction features, usually tire traction is often expected to be somewhat sacrificed as may be evidenced by its decrease in wet and dry skid resistance.
The various tread rubber compositions typically contain carbon black reinforcement. In some instances, a portion of the carbon black might be replaced with silica in which case a coupling agent is used to enhance the reinforcing effect of the silica. A typical reinforcing agent is 3,3'-bis(trimethoxysilylpropyl) polysulfide and is taught in U.S. Pat. Nos. 3,878,489 and 4,278,587.
Various blends of rubbers have been proposed, taught and/or used for rubber tread compositions. For example, rubber blends of various combinations of medium vinyl polybutadiene (containing about 20-50 percent vinyl 1,2-structure), together with styrene/butadiene copolymer, cis 1,4-polyisoprene and/or cis 1,3-polybutadiene rubbers.
Typically carbon black is used for rubber reinforcement. Although an alternate reinforcement in the form of kaolin clay as a replacement for a portion of the carbon black may be desired for various purposes, use of the clay in such a rubber blend of polybutadiene and styrene/butadiene rubber in a tire tread was observed to be unsuccessful because the tire's treadwear was reduced.
In practice, various rubber compositions have been prepared for various purposes, some of which have included tire treads, which compose a polybutadiene containing a degree of 1,2-monomer configuration, sometimes referred to as vinyl content. Representative of such various compositions include those, for example, taught in various patent specifications such as U.S. Pat. No. 3,937,681 relating to a tire tread of polybutadiene containing twenty-five to fifty percent of its monomer units in a 1,2-position, British Pat. No. 1,166,832 relating to a tire tread of "high vinyl" butadiene rubber containing at least fifty percent of 1,2-addition, U.S. Pat. No. 4,192,366 relating to a tire tread of polyisoprene and medium vinyl polybutadiene rubbers, U.S. Pat. No. 4,259,218 relating to a composition of "medium vinyl" polybutadiene and blends thereof with natural rubber where such composition is required to contain a certain carbon black, U.S. Pat. No. 3,978,165 relating to a composition, taught to be useful for tire treads composed of (a) "medium vinyl" polybutadiene, (b) polybutadiene and (c) butadiene/styrene rubbers, German Pat. No. DE 2936-72 relating to mixtures of polybutadiene containing 35-70% of 1,2-units mixed with polyisoprene rubber and, optionally, with cis polybutadiene or styrene-butadiene rubber for tires, and U.S. Pat. Nos. 3,827,991, 4,220,564 and 4,224,197 relating to combinations of polybutadiene containing at least seventy percent of 1,2-configuration with various other rubbers.
Although such rubber compositions are taught to provide various benefits, some for tire treads, it continues to be desirable to provide a pneumatic tire having a rubber tread having an enhanced rolling resistance commensurate with reasonable traction qualities while still providing good treadwear.